Preparation of novel metal oxides/hydroxides materials and their applications in supercapacitors
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Supercapacitors, as next-generation efficient, safe and clean energy storage devices with superior power density, fast charge/discharge rate, excellent temperature adaptability and remarkable service life, significantly balance the conventional capacitors and the batteries and have been adopted in many fields. Compared with electric double-layer capacitors, pseudocapacitors can produce a higher specific capacitance to satisfy the ever-growing demand, consequently becoming the research hotspot. Co3O4, Ni(OH)2 and Co(OH)2, in view of overwhelming theoretical specific capacitances, modest cost and environmental friendliness, have been extensively recognized as the most promising electrode materials and trigged numerous efforts. However, currently, the achieved specific capacitances of Co3O4 electrodes in previous contributions are still lower than the theoretical value. Besides, the generally accompanying poor rate performances further hinder the application. To address the issue, the hollow Co3O4 spheres with high porosity and thin-wall feature were synthesized. The as-fabricated Co3O4 electrode delivers a high specific capacitance of 988 F/g at 1 A/g and only 6.4% of its initial value decays at 20 A/g. Via analysis of the formation mechanism of zeolitic imidazolate framework-67, the size adjustment in preparation of hollow Co3O4 nanoboxes was obtained, which offers novel perspective and approach for optimization of hollow Co3O4. The hollow Co3O4 nanoboxes electrode exhibits large specific capacitances of 1832.7 and 1324.5 F/g at 1 and 20 A/g, respectively. Recently, the Ni-Co binary hydroxide, which outperforms the single hydroxide thanks to the virtues of stronger layer orientation, increased active sites, reduced resistance and so on, have sparked numerous attentions as a promising route to further enhance the performances of Ni(OH)2 and Co(OH)2 based electrodes. Unfortunately, due to undesirable morphology and involved binder, the progress to date is only in partial fulfilment of the requirements of high performance supercapacitors. Aiming at the defect, a morphology-controlled synthesis to grow Ni-Co binary hydroxide on nickel foam directly (binder-free) was proposed, by which excellent overall supercapacitors performances in terms of specific capacitance (2807 F/g at 1 A/g), rate capability (2222 F/g at 20 A/g) and cycling stability, together with comparable loading mass and facile fabricated method were obtained. In addition, the supercapacitors behaviors of flowerlike ZnWO4, a potential alternative electrode material, were investigated.